Liquid developing apparatus of electrostatic latent image

ABSTRACT

A liquid developing apparatus for electrostatic latent images, the apparatus including a developing electrode having open areas where the ratio of open areas to non-open areas decreases along the path of the electrophotographic material.

United States Patent [191 Sato et al.

[ Aug. 27, 1974 1 LIQUID DEVELOPING APPARATUS OF ELECTROSTATIC LATENT IMAGE [75] Inventors: Masamichl Sato; Osamu Wkushima,

both of Asaka, Japan [73] Assignee: Fuji Photo Film Co., Ltd.,

Kanagawa, Japan 22 Filed: Mar. 16, 1971 21 Appl.No.: 124,778

[30] Foreign Application Priority Data [58] Field of Search 118/637, DlG.- 23, 7, 411, 118/412; 117/37 LE, 17.5; 355/10; 204/181 [56] References Cited UNITED STATES PATENTS 3,200,058 8/1965 Oster 204/181 3,345,927 10/1967 Takats 95/89 3,407,786 10/1968 Beyer et a1 118/637 3,472,676 10/ 1969 Cassiers et a1. 117/37 3,577,259 5/1971 Sato et a1 117/37 3,603,289 9/1971 Kasuya 118/637 3,669,073 6/1972 Savit et al. 1 18/637 FOREIGN PATENTS OR APPLICATIONS 1,908,847 9/ 1969 Germany 117/37 LE Primary Examiner-Mervin Stein Assistant Examiner-Leo Millstein Attorney, Agent, or Firm-J. T. Martin; Gerald J. Ferguson, Jr.; Joseph J. Baker [57] ABSTRACT A liquid developing apparatus for electrostatic latent images, the apparatus including a developing electrode having open areas where the ratio of open areas to non-open areas decreases along the path of the electrophotographic material.

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LIQUID DEVELOPING APPARATUS OF ELECTROSTATIC LATENT IMAGE BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of an electrophotographic material of which the right half of the surface thereof is uniformly charged.

FIG. 2 shows the distribution of electrostatic charge on the surface of an electrophotographic material electrostatically charged as shown in FIG. I.

FIGS. 3 and 4 show the distribution in the absence and presence of developing electrode respectively of electric field perpendicular to and in the proximity of the surface of an electrophotographic material provided with the electrostatic charge distribution as shown in FIG. 2.

FIG. 5 shows the distribution of electrostatic charge obtained when-electrostatic charging similar to that in FIG. 1 is applied to the surface of an electrophotographic material provided with uniform retentive charge.

FIG. 6 shows the distribution of electric field in the proximity of the surface provided with the charge distribution shown in FIG. 5 when the effect of developing electrode is large.

FIG. 7 shows the distribution of electrode field when said effect is small.

FIG. 8 shows a distribution of electrostatic charge in which a uniform low density area and a uniform high density area are adjacent.

FIG. 9 shows the distribution of-electric field in the proximity of the surface of an electrophotographic ma terial provided with the charge distribution shown in FIG. 8.

FIG. 10 shows the distribution of electrostatic charge showing gradual change from low charge density to high charge density.

FIG. 11 shows the distribution of electric field generated by the charge distribution shown in FIG, 10.

FIG. 12 is a plane view showing the state of develop ment observed when liquid developer is made to flow on the surface of an electrophotographic material provided with stepwise distribution of electrostatic charge.

FIGS. 13-17 show longitudinal cross-section views of the developing electrodes for use in the apparatus of this invention.

FIG. 18 and FIG. 19 are plane views of developing electrodes for use in the apparatus of this invention.

This invention relates to a developing apparatus for electrophotography, and particularly to a liquid developing apparatus capable of preventing fogging'due to uniform background stain resulting from retentive electrostatic charge, preventing the formation of white non-developed portion (so-called halo) in low density areas adjacent to high desnity areas at the proximity of boundary adjacent thereto, preventing the formation of streaks resembling the tail of comet on the developed image downstreams along the flow of liquid developer, developing a wide area of uniform charge density into a uniform image density thereby preventing socalled edge effect, and enabling developing at a high speed.

In general, the photoconductive insulating layers employed in electrophotography show certain amount of retentive potential. When such photoconductive insulating layer is subjected to imagewise exposure after uniform electrostatic charging, the charge on the areas subjected to light irradiation is dissipated while that on the areas not subjected to light irradiation is retained unchanged on said layer. The amount of charge dissipated becomes larger as the intensity of light becomes stronger to form electrostatic latent image corresponding to the light image. In actual case, however, the dissipation of electrostatic charge does not proceed in accordance with the increase of light intensity and the rate of dissipation decreases when the intensity of light exceeds a certain limit, and electric charge which is ob-= served as so-called retentive potential remains on the areas where the electrostatic charge should be completely dissipated if the exposure is terminated before complete dissipation of electrostatic charge. On the other hand if the amount of exposure to light is made sufficiently large for complete dissipation of retentive potential, the electrostatic charge on the areas where the charge should not be dissipated beyond a certain limit is excessively dissipated thereby rendering it impossible to obtain latent image faithfully corresponding to the original pattem. In current processes, therefore, the imagewise exposure is made so as to form certain retentive potential, and the fogging resulting therefrom is removed by suitable method. Such retentive charge, if developed faithfully, will give rise to a very disagreeable image with uniform fogging on the background.

In order to prevent fogging resulting from retentive potential, it was already proposed to apply a direct current voltage externally to the developing electrode so as to a potential comparable to the retentive potential. This method accompanies, however, following drawback when applied to an automatic developing device in which electrophotographic material is continuously driven. This drawback arises from the fact that the electrophotographic material on which the latent image is formed is displaced under a facing relationship with the developing electrode through liquid developer to cause gradual attenuation of electrostatic charge due to leaking in said liquid developer. More detailedly, the amount of retentive charge present on a certain area of the surface holding the latent image becomes smaller during a period from the moment at which said area enters facing relationship with said developing electrode to the moment at which said area ceases to be in facing relationship with said electrode. Accordingly it is impossible to dissipate completely the retentive potential which is continuously changing as explained above by means of applying a constant external voltage to said developing electrode, and it is therefore impossible to obtain a reproduced image faithful to the original pattern. In this prior ideas, besides, it is necessary to regulate the external voltage according to the species of electrophotographic material employed, since these materials are associated with different retentive potentials. Thus, the prior method involves the drawback of inevitably requiring to measure the retentive potential by suitable means.

Also a troublesome phenomenon, so-called halo phenomenon, is known to be specific to electrophotography. This phenomenon is experienced to appear when a low charge density area and a high charge density are adjacent and simultaneously the charge density shows rapid change at the boundary therebetween. Said halo phenomenon is observed as the white undeveloped portion without toner deposition, lacking reproduction of original pattern on the low charge density area close to said boundary. Furthermore said halo phenomenon is observed more markedly when the distance between the developing electrode and the surface holding the latent image is smaller, and this can be supported theoretically as explained later, though this is seemingly contradictory to the theoretical conjecture.

Anotherdrawback specific to electrophotography is the formation of streak on the developed image, which is oberved as the deposition of toner in comet tail-like shape downstreams along the flow of the liquid developer when said liquid developer is made to flow along the surface holding latent image. This phenomenon also makes it impossible to obtain an image of good quality. This phenomenon is most directly caused by the presence of relative speed component between the surface holding the latent image and the liquid developer parallel to said surface. In practice, however, it is difficult to reduce said speed component to zero, and it is not advantageous to reduce said speed component to a very small value from the standpoint of supply of liquid developer. In actual case, therefore, the presence of speed component of certain magnitude should be accepted. Although the streak phenomenon is related with various factors and the mechanism thereof is not yet clarified, it was already made possible to reduce this phenomenon effectively by means of several methods. Empirically the streak phenomenon becomes more marked when the electrostatic charge on toner particles is smaller, and particularly marked at the boundary where the charge of latent image shows rapid change from high density to low density. It is naturally expected that said streak phenomenon is easily observed when said relative speed component is large, but said phenomenon is empirically known to become more marked, even for a same relative speed, with the ability of developing electrode to increase and to absorb the flux of electric force generating from thesurface holding latent image.

This invention consists, in a developing apparatus in which an electrostatic latent image formed on an insulating layer provided on an electroconductive support material and a developing electrode are kept in facing relationship each other with a small spacing in liquid developer composed of fine charged particles dispersed in an electroinsulative liquid, of providing the developing electrode with a larger open area ratio at the initial stage of developing to reduce the effect of said developing electrode which increases and absorbs the lines of electric force generated from said latent image so as to be insufficient for attracting toner particles and to be free from streaks on the developed image, and of reducing the open area ratio on the developing electrode after the retentive charge is substantially attenuated during the process of developing so as not to leave portions where the charge density of latent image shows rapid change thereby intensifying the effect of said .developing electrode and completing the development in this state. The effect of developing electrode means the effect thereof to increase the number of and to absorb the flux of electric force resulting from the electrostatic charge present on the surface holding said electrostatic latent image. According to this apparatus the drawbacks above-mentioned are to be eliminated, and this apparatus is suitable for high-speed developing apparatus. The distance between the developing electrode and the surface holding latent image may be kept constant through the path of the electrophotographic material.

This invention is based upon the following principle. In case the open area ratio of the developing electrode is retained very small from the start and the distance (hereinafter referred to as 3) between the developing electrode and the surface holding a latent image is small, very small retentive charge would also faithfully be developed resulting in fog, and hollow caused by lack of half tone at low density field at the portion where the charge density is changed suddenly, and socalled edge effect'(this in fact does no harm, as the g is very small, namely, this cannot be recognized visually) is resulted, as the peripheral portion of high density area is developed higher density compared with the inside (mid portion), and streak is apt to appear on an image.

These drawbacks are controlled by using developing electrode with a large open area ratio at the initial developing stage and by using electrode with a small open area ratio at the later stage. At the initial stage, the electric field between said electrode and the surface holding a latent image by the retentive charge on said surface would not enforce the toner particles to said surface, and in the meantime the retentive charge would be attenuated by the leakage through the liquid developer, while the development would stress the peripheral portion of high density area resulting the change of charge density milder. Then if the later development is carried out by using a developing electrode with a small open area ratio, the areas provided with retentive charge scarcely attract toner particles since said retentive charge is already attenuated by means of leaking and does not, therefore, show fogging. Besides in the portions where charge density shows abrupt change, the developing electrode with a smaller open area ratio facing to the surface holding the latent image does not give rise to the formation of inverted electric field and therefore does not show halo phenomenon since said abrupt change is already changed into milder one. Furthermore a faithful reproduction of the original image can be obtained even in the internal portion of the high density area by means of the combination of the primary development with stressed edge effect and the secondary development without edge effect. Furthermore, although the streaks are apt to appear when the effect of developing electrode is large (when the open area ratio is small) and where the charge density shows abrupt change, the formation of such streaks is prevented in the process of this invention in the primary development since the effect of said developing electrode at the spacing between said electrode and the surface holding the latent image is maintained small, and also in the secondary development under increased effect of developing electrode since said abrupt change of charge density is already lost.

In the present invention, the effect of the developing electrode is controlled by means of providing the open areas on a surface of said developing electrode kept in facing relationship with the surface holding latent image thereon. The openarea ratio is a ratio of the area of the opening on the developing electrode. For example, where there is no opening on said electrode said ratio is zero. The open area ratio may be changed on one developing electrode. Or several electrodes which have different open area ratio may be used. The open area ratio is hereinafter referred to as S.

The description thus far made will be explained in reference to attached drawing.

FIG. 1 shows the cross section of an electrophotographic material of which right half of the surface is provided with uniform electrostatic charge. The photoconductive insulating layer 11 is composed of material already known in the prior art, such as amorphous selenium or photoconductive powdered material for example photoconductive zinc oxide powder blended with insulative resinous binder. The electroconductive support material layer 12 can be composed for example of metal plate, or plastic film or paper previously subjected to electroconductive treatment. Said layer 12 is usually grounded during the course of the development. FIG. 1 shows an example of electrostatic latent image which is composed of positive charge provided uniformly on the right half of the surface of said layer 11 and negative charge of same amount induced on the corresponding half of the boundary thereunder.

FIG. 2 graphically shows the distribution of positive charge shown in FIG. I, with the abscissa representing lateral expansion of said surface and with the original point being located at the boundary of the area of electrostatic charge shown in FIG. 1. In this case said electrostatic charge is assumed to be present at a charge density of on the right-hand half of said surface.

FIGS. 3 and 4 are the graphs showing the component perpendicular to the surface holding latent image of the electric field in the vicinity of said surface resulting from the charge distribution shown in FIG. 2. In FIG. 3 showing a case in which developing electrode is absent or the distance between said electrode and the latent image is very large, the distribution of electric field takes a form completely different from that of electrostatic field. This phenomenon is well known as edge effect. In this case it is to be noted that a negative field as shown in FIG. 3 (the electric field is defined positive when the direction thereof is upward from the surface holding the latent image, as shown in FIG. 1) even in the left-hand half of the surface in which 0' O as shown in FIG. 2. Such negative will be hereinafter referred to as inverted electric field. Such inverted electric field is observed to be particularly large in the vicinity of boundary portions.

FIG. 4 is a graph showing the distribution of electric field observed when the surface holding latent image is kept in facing relationship with a grounded developing electrode at a fixed distance therefrom. The curves B y and A inFIG. 4 correspond respectively to the cases where distance between the developing electrode'and the surface holding the latent image (g) are made very small (for example several ten microns) and respectively large (for example 5-10 mm). The distribution of electric field and of electrostatic charge show considerably good coincidence when g is thus very small, though a slight inverted electric field still persists and is particularly large at the vicinity of edge portion (represented by the original point of the graph) compared with that in the areas distant therefrom. Thus invert'ed field still remains large even when g is maintained small. When g is large, the inverted field becomes small. The curve A changes to the curve B when S is made larger even if g is kept at a small value.

FIG. 5 shows the distribution of electrostatic charge yv hen a retentive charge 0., 9f 0 is present on the area (left to the. original point) where 0 is desired to be equal to zero. The charge distribution shown in FIG. 2 is an ideal one whereas actual imagewise exposure provide a'distribution as shown in FIG. 5.

FIG. 6 is a graph showing the component of electric field perpendicular to and in the vicinity of the surface holding latent image of the charge distribution shown in FIG. 5 in facing relationship with said surface with relatively small value of g and s. As shown in FIG. 6, the electric field in the areas where, retentive charge 0' of FIG. 5 is present is positive except the boundary portions of electrostatic charge distribution. In such boundary portions, on the contrary, the electric field shows negative minimum point due to remaining edge effect. If liquid developer (consisting in this case of negatively charged minute particles, or so-called toner, suspended in insulating liquid), is supplied onto the surface holding the latent image in such state of electric field distribution, toner particles, which are attracted by positive electric field, also deposit on the left-hand half of said surface where such deposition is originally not desirable to cause fogging due to retentive charge.

FIG. 7 is a graph showing the distribution of electric field by the charge distribution shown in FIG. 5 when the value of g is maintained relatively small and S is maintained relatively large and uniformly on the electrode. Due to the enlarged S, in this case (corresponding to the curve Bin FIG. 4), the inverted electricfield resulting from the areas with charge density 0 (righthand half) and the positive electric field resulting from the areas with charge density 0 (left-hand half) cancel each other to leave no positive electric field even in the areas with retentive charge. Thus, in this case, no deposition'of toner appears in the left-hand half of the surface.

In the following explanation will be made on the halo phenomenon observed in the prior developing methods. FIG. 8 shows the distribution of electrostatic charge with abrupt change at the boundary between the low density area with charge density a, and the high density area with charge density 0' The solid line in FIG. 9 shows the distribution of electric field when the values of g and S are kept very small in case of the charge distribution shown in FIG. 8. Thus, the deposition of toner appears according to said solid line if development is carried out by supplying liquid developer on the surface holding the latent image while the values of g and S are maintained small. If the development is interrupted in the course thereof, the distribution of remaining charge after neutralization with toner comes to the form shown by the broken line in FIG. 8. Since the values of g and S arevery small in this case, the neutralization of charge proceeds approximately in proportion to the charge distribution thereof except at the boundary portion. Consequently the distribution of electric field resulting from the remaining charge distribution, when the values of g and S are very small, takes the fonn represented by the broken line in FIG. 9, which is essentially similar to that represented by the solid line therein and thus is also provided with negative electric field at the boundary portions. As the conclusion, when the developing electrode with very small values of g and S from the start of development the inverted electric field is maintained throughout the course of development at the proximity of boundary portions where charge density shows abrupt charge, thus preventing the deposition of toner in said portions to result in socalled halo.

FIG. 10 shows the distribution of remaining electrostatic charge obtained when the development is carried out on the charge distribution represented by the solid line in FIG. 8 with relatively large value of S and relatively small value of g (for example ca. several hundred p.) and is terminated in the course thereof. The initial distribution of electric field takes the form of FIG. 7 due to large S, and thus the deposition of toner perferentially starts from the areas with stronger electric field in the vicinity of boundaries.

Such deposition neutralizes the electrostatic charge exclusively in the portions where the electric field is strong, whereas the electrostatic charge in the portions distant from the boundaries remains almost intact. Strictly speaking, however, the charge shows a slight attenuation due to leaking through the liquid developer during said development, resulting in a milder charge distribution as shown in FIG. 10. The distribution of electric field changes to the form of FIG. 11, no longer showing inverted electric field due to the absence of abrupt change in electrostatic charge distribution.

H6. 11 shows the distribution of electric field corresponding the charge distribution of FIG. 10 when the value of g and S are kept very small. In this case the value of g and/or S can be made as small as possible within practical range because the inverted electric field as shown in FIG. 9 is no longer formed in this case due to the absence of abrupt change of charge density even when the value of g is maintained very small. Accordingly a mild distribution of electric field as shown in FIG. ll hardly accompanies halo or streaks.

Although the fogging andstreaks are advantageously reduced by maintaining the value of S at a large value throughout the development, such operation should be prevented because the developing speed is extremely lowered and because it is impossible to obtain a satisfactory image faithful to the latent image and free from the edge effect. Consequently it is necessary to decrease the value of S after a suitable time. Said time is generally in the range of several ten seconds to several hundreds seconds and sometimes exceeds this range, though said time is strongly dependent on various factors such as characteristics of electrophotographic material, resistance of liquid developer, electrostatic charge on toner particles concentration of toner particles in the liquid developer, migrating speed of said toners, etc.

F i0. 12 shows the influence of the value of g and S on the phenomenon of streak, and shows the result of development when the liquid developer is made to flow in the direction of arrow on electrostatic charge distributed in stepwedge shape under extremely small values of g and S. The developed image experimentally shows light long streaks and dark short streaks in the direction of flow of the liquid developer. When the development is carried out with relatively small value of g and large value of S, said short streaks vanished while said long streaks persists. Said long streaks can be also removed when the flow speed of the liquid developer is made smaller. Further, said long streaks vanish when the flow speed of liquid developer is made smaller regardless of the value of g and S, but said short streaks persist unless the value of g or S is made larger. Also it is confirmed that said short streaks no longer appear even if the value of g or S is made larger during the course of the development.

For example it is experimentally confirmed that said dark short streaks do not appear when the development is carried out for 0.3-5 seconds with the value'of g of several hundred microns and with the large value of S.

The scope and spirit of the present invention thus far explained can be summarized as follows:

1. Fogging:

Since it is practically impossible to prevent the formation of retentive potential in electrophotography, the formation of fogging due to the electric field resulting from retentive potential is inevitable if the development is carried out under the small open area ratio and distance between the surface holding latent image and developing electrode from the start of said development. It is possible, however, to remove said fogging by keeping the open area ratio large at the initial stage of said development to attenuate said retentive potential under the small distance between the developing electrode and the electrostatic latent image.

2. Halo and edge effect in the image areas:

ln electrophotography so-called halo or toner lacking portions appear in the low density portions close to the boundaries where the charge density shows abrupt change, and the high charge density areas at said boundaries show higher image density compared with that in the internal portions in said areas to give rise to so-called edge effect. These phenomena are theoretically inevitable unless the distance between the developing electrode and the electrostatic latent image is reduced to zero. On the other hand, according to this invention, it is possible to reduce said edge effect to a practically acceptable level by developing under large open area ratio of developing electrode at first and then under small open area ratio.

3. Streaks:

The developed image shows streaks when the portions where charge density shows abrupt change are developed with liquid developer provided with a relative speed component with respect to the latent image, and

such phenonmenon is more easily formed for a fixed relative speed when the open area ratio or the distance between the electrode and the surface holding latent image is smaller, or when the change of charge density is more vigorous. It is possible to prevent the formation of such streaks by effecting development at first with increased open area ratio and even with small distance between the developing electrode and the latent image thereby giving stress to the edge portions and rendering the change of charge density milder, and then by decreasing open area ratio.

In the following explained are some samples of the devices for realizing the process according to this invention.

FIG. 13 is a schematic cross sectional view of an example of the device embodying the present invention in which provided is a grounded flat metal developing electrode 1. This developing electrode 1 comprises several thin narrow strip type electrodes 2-9 arranged at right angles to the path of electrophotographic material with crevices (openings) 14-19 among them. An endless belt 20 made of metal plate, metal mesh, plastic material, rubber, cloth etc., is driven around two rollers 21 and 22. Electrophotographic sheet material 10 (hereinafter referred to as sheet) provided with electrostatic latent image thereon is supported by said belt 20 and transported from right to left. The electroconductive layer of said sheet 10 is grounded separately or by means of said belt 20. The sheet supporting surface of said belt is inclined a little with respect to the horizontal surface so as that the liquid developer flows from right to left. The liquid developer is stored in a container 23, supplied to the pipe through the outlet 24 of said container 23 by means of a pump (not shown) and fed onto the sheet 10 through the openings or slits provided on said pipe 25. All the electrodes 2, 3, 4, 5, 6, 7, 8, 9 are of same size and arranged at the same distance from the surface of said sheet 10. The crevices l3, l4, 15, 16, 17, 18 and 19 between said electrodes are made gradually smaller with the path of said sheet. That is, the crevice 13 is largest and 19 is smallest. Each electrode surface is parallel to and nearly rectangular to the path of said sheet. As for the developing electrode 1, plural electrodes as aforementioned or a piece of metal plate provided with openings corresponding to said crevices 13, 14, 15, l6, 17, 18 and 19 may be used. In case plural electrodes are used, it is preferable to electrically connect these electrodes. These electrodes may be electrically biased in different potential each other. For example, when the retentive charge of latent image is large and the charge will not sufficiently attenuate due to leakage through the liquid developer, applying bias voltage to the electrodes is necessary. ln this case it is preferable to gradually decrease said bias voltage, because the retentive charge attenuates as the development proceeds.

ln the apparatus shown in FIG. 13, thelength of the electrode strips along the path of the sheet is practically from about one-tenth millimeters to several ten centimeters and preferably several millimeters to several centimeters. The longitudinal cross-sectional figure of these electrode strips, shown in rectangle in FIG. 13, may be circular, oval, square, triangular, etc.

F 1G. 14 shows another embodiment of the developing electrode used in this invention. 22a, 23a, 24a, 25, 26, 27, 28 and 29 denote strip-shaped electrodes the lengths of which along the sheet path are larger in this numerical order. 33, 34, 35, 36, 37, 38 and 39 denote the distance between these electrodes and are kept approximately constant. That is, the open area ratio S is larger on the right hand and is smaller on the left hand in this figure.

FIG. 15 shows the other embodiment of the developing electrode. Electrode strips 42, 43, 44, 45, 46, 47, 48 49 and 50 have the same size, and strip 51 has a longer size to the direction of sheet path. The crevices 52, 53, 54, 55, 56, 57, 58, 59, and 60 between said strips are made gradually smaller. The ratio S is equal to zero at the strip 51. I

FIG. 16 shows an example of a device employing a semicylindrical developing electrode, in which a flexible electrophotographic web material 10 is driven continuously at a constant speed around the drum 70 with the latent image outwards. Electrode strips 61, 62, 63, 64, 65, 66, 67, 68 and 69 are arranged semicylindrically at a constant distance from the surface of said material, with the crevices 72-79. The crevices 72-79 are gradually decrease in this order. 71 denotes the level of liquid developer.

FIG. 17 shows an embodiment of the developing electrode, wherein five metal wires are used. 80-89 denote wire electrodes tightened in parallel. .9l-99 denote the distances among neighboring electrodes. These distances decrease in numerical order.

Although the examples thus far cited relate to the case where numbers of electrode strips or wires have been from eight to sixteen, this invention can be realized by more or less electrodes.

The distance g between the surface holding latent image and the developing electrode is not necessary constant. The open area ratio S may partly become large during developing process.

In this invention S is large in initial stage of the development, then exchange or stirring of liquid developer between the surface and developing electrode will be done with ease and resulting fast developing speed.

FIGS. 18 and 19 show the other embodiments of developing electrode. The developing electrode shown in FIG. 18 is a metal flat plate 100 provided with rectangular perforations. FIG. 19 shows one provided with circular perforations. In both developing electrode, S is changed smaller from right to left.

The developing electrode to be employed in this invention is not necessarily required to be grounded. Grounded electrode tends to lead to fogging due to retentive potential. It is also possible not to ground said electrode in the initial stage of development and to ground exclusively in the latter stage thereof.

As thus far explained, it is made possible, according to this invention, to make development under reduced effect of the developing electrode absorbing the line of electric force causedby latent image using a developing electrode with a large open area'ratio in initial stage, and then under increased effect of developing electrode using a developing electrode with a small open area ratio.

When the retentive charge is not sufficiently reduced while the abrupt change of charge density is made milder in initial stage, it is preferable to apply a bias voltage to cause toner particles away from the latent image for preventing fogging.

-According to this invention, an electrophotographic image without fogging, halo, and edge effect is obtained and high speed development is possible.

What is claimed is:

1. A liquid developing apparatus for developing electrostatic latent images with a developing electrode having open areas and non-open areas, the ratio of open areas to non-open areas substantially continuously decreasing along the path of a moving electrophotographic sheet material containing an electrostatic latent image, the rate of decrease of said ratio being a function of the rate of decay of retentive, image background charge on said electrophotographic material such that said ratio is a first value when said retentive, background charge has a first value so that little, if any, of said retentive charge is developed to thereby substantially avoid background fogging and said ratio being a second value, substantially less than said first value of the ratio, after said retentive charge has decayed to a second value substantially less than said first value of the retentive charge so that development of said latent image may be completed with little, if any, background fog formation, streak formation, edge or halo effect.

2. An apparatus according to claim 1 wherein said developing electrode comprises a conductive plate having openings whose ratio to the non-open areas on the electrode is decreased along the path of the electrophotographic liquid developing material.

3. A liquid developing apparatus as in claim 1 where said developing electrode comprises a conductive plate and said open areas comprise circular openings in said plate.

4. A liquid developing apparatus for developing electrostatic latent images on electrophotographic material with a developing electrode having open areas and nonopen areas, the ratio of open areas to non-open areas decreasing along the path of the electrophotographic liquid developing material, said developing electrode comprising conductive narrow strips arranged at right angles to the path of the electrophotographic liquid developing material with openings between said strips along said paths.

5. An apparatus according to claim 4 wherein said strips are of the same width and said openings are decreased along said path.

6. An apparatus according to claim 4 wherein the width of said strips is increased along said path and the openings are same.

7. A liquid developing apparatus for developing electrostatic latent images on electrophotographic material with a developing electrode having open areas and nonopen areas, the ratio of open areas to non-open areas decreasing along the path of the electrophotographic liquid developing material, said developing electrode comprising a conductive plate having openings whose area ratio on the electrode is decreased along the path of electrophotographic material where the configuration of said openings is rectangular, the longer dimension of each rectangle being aligned at a right angle to said path.

v 8. A liquid developing apparatus for developing electrostatic latent images on electrophotographic material with a developing electrode having open areas and nonopen areas, the ratio of open areas to non-open areas decreasing along the path of the electrophotographic liquid developing material, said developing electrode comprising a plurality of wire electrodes disposed in parallel, the distance between successive wire elec trodes decreasing along the path of electrophotographic liquid developing material. 

1. A liquid developing apparatus for developing electrostatic latent images with a developing electrode having open areas and non-open areas, the ratio of open areas to non-open areas substantially continuously decreasing along the path of a moving electrophotographic sheet material containing an electrostatic latent image, the rate of decrease of said ratio being a function of the rate of decay of retentive, image background charge on said electrophotographic material such that said ratio is a first value when said retentive, background charge has a first value so that little, if any, of said retentive charge is developed to thereby substantially avoid background fogging and said ratio being a second value, substantially less than said first value of the ratio, after said retentive charge has decayed to a second value substantially less than said first value of the retentive charge so that development of said latent image may be completed with little, if any, background fog formation, streak formation, edge or halo effect.
 2. An apparatus according to claim 1 wherein said developing electrode comprises a conductive plate having openings whose ratio to the non-open areas on the electrode is decreased along the path of The electrophotographic liquid developing material.
 3. A liquid developing apparatus as in claim 1 where said developing electrode comprises a conductive plate and said open areas comprise circular openings in said plate.
 4. A liquid developing apparatus for developing electrostatic latent images on electrophotographic material with a developing electrode having open areas and non-open areas, the ratio of open areas to non-open areas decreasing along the path of the electrophotographic liquid developing material, said developing electrode comprising conductive narrow strips arranged at right angles to the path of the electrophotographic liquid developing material with openings between said strips along said paths.
 5. An apparatus according to claim 4 wherein said strips are of the same width and said openings are decreased along said path.
 6. An apparatus according to claim 4 wherein the width of said strips is increased along said path and the openings are same.
 7. A liquid developing apparatus for developing electrostatic latent images on electrophotographic material with a developing electrode having open areas and non-open areas, the ratio of open areas to non-open areas decreasing along the path of the electrophotographic liquid developing material, said developing electrode comprising a conductive plate having openings whose area ratio on the electrode is decreased along the path of electrophotographic material where the configuration of said openings is rectangular, the longer dimension of each rectangle being aligned at a right angle to said path.
 8. A liquid developing apparatus for developing electrostatic latent images on electrophotographic material with a developing electrode having open areas and non-open areas, the ratio of open areas to non-open areas decreasing along the path of the electrophotographic liquid developing material, said developing electrode comprising a plurality of wire electrodes disposed in parallel, the distance between successive wire electrodes decreasing along the path of electrophotographic liquid developing material. 